1. Field of the Invention
The present invention relates to a gas turbine engine, and more specifically to a hybrid gas turbine and internal combustion engine.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
Historically, gas turbines have not been competitive with other internal combustion engines in applications where a large portion of the mission is spent at power levels significantly below maximum power, and where their inherent power density and emissions advantages are not substantially valued. This is due to the steep falloff in gas turbine thermal efficiency as power is reduced, resulting in unacceptable low-power fuel consumption. This shortcoming has historically been addressed by the addition of a recuperator, which not only increases thermal efficiency at maximum load, but also reduces the efficiency lapse rate as load is reduced. In addition to the recuperator, variable power turbine inlet guide vanes are sometimes also included to further reduce the efficiency lapse.
Still, even the recuperated gas turbine with variable power turbine inlet guide vanes realizes a severe thermal efficiency lapse when throttled from maximum load to low power settings. This is primarily the result of reductions in both cycle pressure ratio and turbine inlet temperature. The optimal throttling scheme typically consists of first shutting power turbine variable guide vanes (VGVs), and then reducing turbine inlet temperature (TIT). Gas generator shaft speed is reduced during throttling, resulting in corresponding reductions in flow and pressure ratio (per the gas generator operating line) Reduction in turbomachinery efficiency as power is reduced further exacerbates the problem.
Even ignoring the above problems, recuperators themselves have other issues preventing their widespread assimilation into the power generation markets. They are typically large and heavy, inhibiting their use in vehicular applications. Additionally, due to thermal stresses caused by cyclic duty (i.e. intermittent duty on-off operation and excursions from low to high power), recuperators are prone to fatigue and cracking, resulting in leaks which severely impact performance. The result is that recuperators are typically costly and difficult to manufacture, requiring large amounts of welding and many fabrication steps.
For vehicular power, the incumbent gasoline and diesel piston engines represent refinement of these designs over many decades of development. However, these engines exhibit inherently low power density, and the throughput of working fluid (air) is generally very low in comparison to the weight and volume of the engine. Furthermore, these engines produce large concentrations of harmful emissions such as nitrogen oxides, carbon monoxide, and unburned hydrocarbons, and require that a large amount of costly emission reduction equipment be added to the engine system to reduce emissions to acceptable levels. Even then, emission levels are far worse that those of gas turbines.
As federal regulations continue to tighten emission requirements for gas and diesel engines, more esoteric and costly technologies are required, such as direct-injection and variable valve timing. These technologies, while helpful to reducing emissions, add to engine cost and complexity.
Another approach finding its way into the vehicular engine market is that of the hybrid engine. Typically, this entails using a small internal combustion engine that provides the average power needed by the vehicle (typically 10-30% of maximum power demand), along with the use of an energy storage device, typically a bank of batteries, to provide maximum power for short periods of time during the mission. While substantial improvements in vehicle fuel economy and range can be realized by such systems, the required battery banks are generally very voluminous and heavy while adding cost, resulting in only small savings in vehicle weight, and a substantial increase in cost. A typical battery arrangement in a hybrid vehicle might weight 1000 lbs. while the vehicle without the battery would weight 2000 lbs. Eliminating the battery would reduce the vehicle weight by one third.
It is therefore an object of the present invention to improve the overall efficiency of a vehicle power plant.
It is another object of the present invention to reduce the weight of the vehicle power plant without reducing the power produced by eliminating the need for heavy batteries.
It is still another object of the present invention to improve the response time for a gas turbine engine by spinning the turbine while the gas turbine engine is not producing power.